Grokking Microservices for System Design Interviews

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Querying Across Services

1. The Missing JOIN

"Show me this customer's orders, with product names and current shipping status."

In the monolith that is one query with three JOINs. A JOIN is the database operation that stitches rows from several tables into one result. One query, done.

Now split the system. Orders, product names, and shipping statuses live in three private databases. No single engine can see all three, so there is nothing left to run the JOIN. Cross-service reads are the daily tax of this architecture.

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